docs: add vendor ID allocation table

[alexcekay]

Context

The vendor range [20000, 21000) has grown organically with no tracking of who uses what. The result is vendors picking IDs ad-hoc. Without documented allocations, downstream projects cannot add messages independently without risking ID collisions when upstreaming their changes.

Changes

This PR documents current usage and formalizes contiguous blocks so each namespace has room to grow autonomously: 100 IDs for open-source projects, 20 IDs for companies. Blocks are anchored to each vendor's first existing allocation. IDs that fall outside their natural block are listed in another table.

This also adds the px4.* namespace (20800–20899) following the existing ardupilot/ precedent, and com.auterion.* (20900–20919) as Auterion's company namespace.

If your namespace is represented incorrectly or you'd prefer a different block size, comment and I'll happily adapt.

[hamishwillee]

@peterbarker Can we get this particular case looked at please?

[peterbarker]

Doesn't this need to come along with some sort of an allocation policy/mechanism, similar to board_types.txt?